Courtesy of the extension of LARS service from Farnborough, extra radar feeds mean that you can be seen on radar down to about 1500ft or lower in the vicinity of Goodwood.(can see traffic just above ground level at Shoreham on the same radar head) However it's a good idea as Foxy Loxy says to call the relevant FISO unit even when transitting above an ATZ especially when there's large inviting VOR on the airfield which people might be using to carry out IF in VMC; there might also be someone aerobatting over the airfield (as happens at Popham) so if you're all on the same frequency it can only be safer.